I did this:
1. Went to "iPhone" menu (either top right at the bar near the "iTunes Store" or at the side menu on the left, if You have it on).
2. In the iPhone menu I went to "Music".
3. I clicked "Sync Music", it asked to confirm or sth like that... I did not confirm.
4. I selected "Selected playlists, artists, albums, and genres".
5. I approved that selection, don't remember how exactly.
6. This caused erasing all of my entire iPhone music content and assigning my iPhone to my music library ( let's say wow! ;).
7. Now I can put all my music from iTunes library to my iPhone freely, no harm to my Itunes library was made.
It works just like it did before - drag and drop.
Just one thing:::::if You have anything in Your iPhone music that You don't have at the iTunes library, You will most probably loose it. So beware.
p.s.
I needed one or two drinks before I found enough courage to do this above and not to feel too much sorry for the content I thought I could loose. A success.
Hope this helps some of You.
Apple! Better is the worst enemy of good. Remember that.